Re: [patch 3/5] x86: lockless get_user_pages_fast

[Nick Piggin <npiggin@suse.de>]

On Thu, May 29, 2008 at 12:20:59PM -0500, Dave Kleikamp wrote:
> On Thu, 2008-05-29 at 22:20 +1000, npiggin@suse.de wrote:
>  
> > +int get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages)
> > +{
> > +	struct mm_struct *mm = current->mm;
> > +	unsigned long end = start + (nr_pages << PAGE_SHIFT);
> > +	unsigned long addr = start;
> > +	unsigned long next;
> > +	pgd_t *pgdp;
> > +	int nr = 0;
> > +
> > +	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
> > +					start, nr_pages*PAGE_SIZE)))
> > +		goto slow_irqon;
> > +
> > +	/*
> > +	 * XXX: batch / limit 'nr', to avoid large irq off latency
> > +	 * needs some instrumenting to determine the common sizes used by
> > +	 * important workloads (eg. DB2), and whether limiting the batch size
> > +	 * will decrease performance.
> > +	 *
> > +	 * It seems like we're in the clear for the moment. Direct-IO is
> > +	 * the main guy that batches up lots of get_user_pages, and even
> > +	 * they are limited to 64-at-a-time which is not so many.
> > +	 */
> > +	/*
> > +	 * This doesn't prevent pagetable teardown, but does prevent
> > +	 * the pagetables and pages from being freed on x86.
> > +	 *
> > +	 * So long as we atomically load page table pointers versus teardown
> > +	 * (which we do on x86, with the above PAE exception), we can follow the
> > +	 * address down to the the page and take a ref on it.
> > +	 */
> > +	local_irq_disable();
> > +	pgdp = pgd_offset(mm, addr);
> > +	do {
> > +		pgd_t pgd = *pgdp;
> > +
> > +		next = pgd_addr_end(addr, end);
> > +		if (pgd_none(pgd))
> > +			goto slow;
> > +		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
> > +			goto slow;
> > +	} while (pgdp++, addr = next, addr != end);
> > +	local_irq_enable();
> > +
> > +	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
> > +	return nr;
> > +
> > +	{
> > +		int i, ret;
> > +
> > +slow:
> > +		local_irq_enable();
> > +slow_irqon:
> > +		/* Try to get the remaining pages with get_user_pages */
> > +		start += nr << PAGE_SHIFT;
> > +		pgaes += nr;
> 
> Typo: s/pgaes/pages/

Gah, missing quilt refresh. Sorry.

I actually did stick a printk in here and manage to hit this path with
a constructed test case (and with nr ! always = 0 to boot). It seemed
to work fine.

BTW. Andy, I dropped your Reviewed-by: Andy Whitcroft <apw@shadowen.org>
because I did make a couple of these little changes that technically
you hadn't reviewed. I don't know what the exact protocol is regarding
the fluidity of RB/AB... 

---

x86: lockless get_user_pages_fast

Implement get_user_pages_fast without locking in the fastpath on x86.

Do an optimistic lockless pagetable walk, without taking mmap_sem or any page
table locks or even mmap_sem. Page table existence is guaranteed by turning
interrupts off (combined with the fact that we're always looking up the current
mm, means we can do the lockless page table walk within the constraints of the
TLB shootdown design). Basically we can do this lockless pagetable walk in a
similar manner to the way the CPU's pagetable walker does not have to take any
locks to find present ptes.

This patch (combined with the subsequent ones to convert direct IO to use it)
was found to give about 10% performance improvement on a 2 socket 8 core Intel
Xeon system running an OLTP workload on DB2 v9.5

 "To test the effects of the patch, an OLTP workload was run on an IBM
 x3850 M2 server with 2 processors (quad-core Intel Xeon processors at
 2.93 GHz) using IBM DB2 v9.5 running Linux 2.6.24rc7 kernel. Comparing
 runs with and without the patch resulted in an overall performance
 benefit of ~9.8%. Correspondingly, oprofiles showed that samples from
 __up_read and __down_read routines that is seen during thread contention
 for system resources was reduced from 2.8% down to .05%. Monitoring
 the /proc/vmstat output from the patched run showed that the counter for
 fast_gup contained a very high number while the fast_gup_slow value was
 zero."

(fast_gup is the old name for get_user_pages_fast, fast_gup_slow is a counter
we had for the number of times the slowpath was invoked).

The main reason for the improvement is that DB2 has multiple threads each
issuing direct-IO. Direct-IO uses get_user_pages, and thus the threads
contend the mmap_sem cacheline, and can also contend on page table locks.

I would anticipate larger performance gains on larger systems, however I
think DB2 uses an adaptive mix of threads and processes, so it could be
that thread contention remains pretty constant as machine size increases.
In which case, we stuck with "only" a 10% gain.

The downside of using get_user_pages_fast is that if there is not a pte with
the correct permissions for the access, we end up falling back to
get_user_pages and so the get_user_pages_fast is a bit of extra work. However
this should not be the common case in most performance critical code.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: shaggy@austin.ibm.com
Cc: linux-mm@kvack.org
Cc: linux-arch@vger.kernel.org
Cc: apw@shadowen.org

---
 arch/x86/mm/Makefile      |    2 
 arch/x86/mm/gup.c         |  254 ++++++++++++++++++++++++++++++++++++++++++++++
 include/asm-x86/uaccess.h |    3 
 3 files changed, 258 insertions(+), 1 deletion(-)

Index: linux-2.6/arch/x86/mm/Makefile
===================================================================
--- linux-2.6.orig/arch/x86/mm/Makefile
+++ linux-2.6/arch/x86/mm/Makefile
@@ -1,5 +1,5 @@
 obj-y	:=  init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
-	    pat.o pgtable.o
+	    pat.o pgtable.o gup.o
 
 obj-$(CONFIG_X86_32)		+= pgtable_32.o
 
Index: linux-2.6/arch/x86/mm/gup.c
===================================================================
--- /dev/null
+++ linux-2.6/arch/x86/mm/gup.c
@@ -0,0 +1,254 @@
+/*
+ * Lockless get_user_pages_fast for x86
+ *
+ * Copyright (C) 2008 Nick Piggin
+ * Copyright (C) 2008 Novell Inc.
+ */
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/vmstat.h>
+#include <asm/pgtable.h>
+
+static inline pte_t gup_get_pte(pte_t *ptep)
+{
+#ifndef CONFIG_X86_PAE
+	return *ptep;
+#else
+	/*
+	 * With get_user_pages_fast, we walk down the pagetables without taking
+	 * any locks.  For this we would like to load the pointers atoimcally,
+	 * but that is not possible (without expensive cmpxchg8b) on PAE.  What
+	 * we do have is the guarantee that a pte will only either go from not
+	 * present to present, or present to not present or both -- it will not
+	 * switch to a completely different present page without a TLB flush in
+	 * between; something that we are blocking by holding interrupts off.
+	 *
+	 * Setting ptes from not present to present goes:
+	 * ptep->pte_high = h;
+	 * smp_wmb();
+	 * ptep->pte_low = l;
+	 *
+	 * And present to not present goes:
+	 * ptep->pte_low = 0;
+	 * smp_wmb();
+	 * ptep->pte_high = 0;
+	 *
+	 * We must ensure here that the load of pte_low sees l iff pte_high
+	 * sees h. We load pte_high *after* loading pte_low, which ensures we
+	 * don't see an older value of pte_high.  *Then* we recheck pte_low,
+	 * which ensures that we haven't picked up a changed pte high. We might
+	 * have got rubbish values from pte_low and pte_high, but we are
+	 * guaranteed that pte_low will not have the present bit set *unless*
+	 * it is 'l'. And get_user_pages_fast only operates on present ptes, so
+	 * we're safe.
+	 *
+	 * gup_get_pte should not be used or copied outside gup.c without being
+	 * very careful -- it does not atomically load the pte or anything that
+	 * is likely to be useful for you.
+	 */
+	pte_t pte;
+
+retry:
+	pte.pte_low = ptep->pte_low;
+	smp_rmb();
+	pte.pte_high = ptep->pte_high;
+	smp_rmb();
+	if (unlikely(pte.pte_low != ptep->pte_low))
+		goto retry;
+
+	return pte;
+#endif
+}
+
+/*
+ * The performance critical leaf functions are made noinline otherwise gcc
+ * inlines everything into a single function which results in too much
+ * register pressure.
+ */
+static noinline int gup_pte_range(pmd_t pmd, unsigned long addr,
+		unsigned long end, int write, struct page **pages, int *nr)
+{
+	unsigned long mask;
+	pte_t *ptep;
+
+	mask = _PAGE_PRESENT|_PAGE_USER;
+	if (write)
+		mask |= _PAGE_RW;
+
+	ptep = pte_offset_map(&pmd, addr);
+	do {
+		pte_t pte = gup_get_pte(ptep);
+		struct page *page;
+
+		if ((pte_val(pte) & (mask | _PAGE_SPECIAL)) != mask) {
+			pte_unmap(ptep);
+			return 0;
+		}
+		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+		page = pte_page(pte);
+		get_page(page);
+		pages[*nr] = page;
+		(*nr)++;
+
+	} while (ptep++, addr += PAGE_SIZE, addr != end);
+	pte_unmap(ptep - 1);
+
+	return 1;
+}
+
+static inline void get_head_page_multiple(struct page *page, int nr)
+{
+	VM_BUG_ON(page != compound_head(page));
+	VM_BUG_ON(page_count(page) == 0);
+	atomic_add(nr, &page->_count);
+}
+
+static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
+		unsigned long end, int write, struct page **pages, int *nr)
+{
+	unsigned long mask;
+	pte_t pte = *(pte_t *)&pmd;
+	struct page *head, *page;
+	int refs;
+
+	mask = _PAGE_PRESENT|_PAGE_USER;
+	if (write)
+		mask |= _PAGE_RW;
+	if ((pte_val(pte) & mask) != mask)
+		return 0;
+	/* hugepages are never "special" */
+	VM_BUG_ON(pte_val(pte) & _PAGE_SPECIAL);
+	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
+
+	refs = 0;
+	head = pte_page(pte);
+	page = head + ((addr & ~HPAGE_MASK) >> PAGE_SHIFT);
+	do {
+		VM_BUG_ON(compound_head(page) != head);
+		pages[*nr] = page;
+		(*nr)++;
+		page++;
+		refs++;
+	} while (addr += PAGE_SIZE, addr != end);
+	get_head_page_multiple(head, refs);
+
+	return 1;
+}
+
+static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
+		int write, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pmd_t *pmdp;
+
+	pmdp = pmd_offset(&pud, addr);
+	do {
+		pmd_t pmd = *pmdp;
+
+		next = pmd_addr_end(addr, end);
+		if (pmd_none(pmd))
+			return 0;
+		if (unlikely(pmd_large(pmd))) {
+			if (!gup_huge_pmd(pmd, addr, next, write, pages, nr))
+				return 0;
+		} else {
+			if (!gup_pte_range(pmd, addr, next, write, pages, nr))
+				return 0;
+		}
+	} while (pmdp++, addr = next, addr != end);
+
+	return 1;
+}
+
+static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end, int write, struct page **pages, int *nr)
+{
+	unsigned long next;
+	pud_t *pudp;
+
+	pudp = pud_offset(&pgd, addr);
+	do {
+		pud_t pud = *pudp;
+
+		next = pud_addr_end(addr, end);
+		if (pud_none(pud))
+			return 0;
+		if (!gup_pmd_range(pud, addr, next, write, pages, nr))
+			return 0;
+	} while (pudp++, addr = next, addr != end);
+
+	return 1;
+}
+
+int get_user_pages_fast(unsigned long start, int nr_pages, int write, struct page **pages)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long end = start + (nr_pages << PAGE_SHIFT);
+	unsigned long addr = start;
+	unsigned long next;
+	pgd_t *pgdp;
+	int nr = 0;
+
+	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
+					start, nr_pages*PAGE_SIZE)))
+		goto slow_irqon;
+
+	/*
+	 * XXX: batch / limit 'nr', to avoid large irq off latency
+	 * needs some instrumenting to determine the common sizes used by
+	 * important workloads (eg. DB2), and whether limiting the batch size
+	 * will decrease performance.
+	 *
+	 * It seems like we're in the clear for the moment. Direct-IO is
+	 * the main guy that batches up lots of get_user_pages, and even
+	 * they are limited to 64-at-a-time which is not so many.
+	 */
+	/*
+	 * This doesn't prevent pagetable teardown, but does prevent
+	 * the pagetables and pages from being freed on x86.
+	 *
+	 * So long as we atomically load page table pointers versus teardown
+	 * (which we do on x86, with the above PAE exception), we can follow the
+	 * address down to the the page and take a ref on it.
+	 */
+	local_irq_disable();
+	pgdp = pgd_offset(mm, addr);
+	do {
+		pgd_t pgd = *pgdp;
+
+		next = pgd_addr_end(addr, end);
+		if (pgd_none(pgd))
+			goto slow;
+		if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
+			goto slow;
+	} while (pgdp++, addr = next, addr != end);
+	local_irq_enable();
+
+	VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
+	return nr;
+
+	{
+		int i, ret;
+
+slow:
+		local_irq_enable();
+slow_irqon:
+		/* Try to get the remaining pages with get_user_pages */
+		start += nr << PAGE_SHIFT;
+		pages += nr;
+
+		down_read(&mm->mmap_sem);
+		ret = get_user_pages(current, mm, start,
+			(end - start) >> PAGE_SHIFT, write, 0, pages, NULL);
+		up_read(&mm->mmap_sem);
+
+		/* Have to be a bit careful with return values */
+		if (nr > 0) {
+			if (ret < 0)
+				ret = nr;
+			else
+				ret += nr;
+		}
+
+		return ret;
+	}
+}
Index: linux-2.6/include/asm-x86/uaccess.h
===================================================================
--- linux-2.6.orig/include/asm-x86/uaccess.h
+++ linux-2.6/include/asm-x86/uaccess.h
@@ -3,3 +3,6 @@
 #else
 # include "uaccess_64.h"
 #endif
+
+#define __HAVE_ARCH_GET_USER_PAGES_FAST
+

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